HISTORY LESSON: Crowds (above) listen to President Obama speak yesterday on the spot where King delivered his historic address 50 years ago. (Getty Images)

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President Obama honored Dr. Martin Luther King Jr. yesterday — the 50th anniversary of King’s “I have a dream” speech — by urging “constant vigilance” to ensure the progress made by the civil-rights movement.

Standing on the very spot in front of the Lincoln Memorial where King stood as he spoke on Aug. 28, 1963, Obama said the Nobel-winning preacher and other slain 1960s activists for racial equality “did not die in vain.”

“Their victory was great,” the nation’s first black president said.

“But we would dishonor those heroes as well to suggest that the work of this nation is somehow complete.”

Tens of thousands of people gathered on a rainy day for the ceremony, in contrast to the glorious 1963 summer day when an estimated 250,000 took part in the historic March on Washington.

“We may never duplicate the swelling crowds and dazzling procession of that day so long ago — no one can match King’s brilliance,” Obama said.

He added an appeal to “stand together for good jobs and just wages.”

Former President Bill Clinton said the March on Washington and King’s speech had “changed America” — and himself.

“They opened minds, they melted hearts and they moved millions — including a 17-year-old boy watching alone in his home in Arkansas,” Clinton said.

Other celebrities who spoke included Oprah Winfrey, Jamie Foxx and Forest Whitaker, who urged the crowd to emulate the “silent heroes and heroines of the movement.”

At 3 p.m., the exact hour of King’s address, members of his family answered his call to “let freedom ring” by tolling the bell that once hung in the Birmingham, Ala., church bombed by members of the Ku Klux Klan in September 1963, killing four black girls.

In Times Square, about 200 people watched Obama’s speech on a giant screen outside the Marriott Marquis.

“I feel like Obama has watched a lot of Dr. King’s speeches,” said Sarah Powers, 50. “Like him, Obama knows how to pause and let us feel . . . what he’s saying.”

In Harlem, Mike Sommerville, 66, watched the president’s speech at home before heading out to sell cooked seafood at the corner of West 132nd Street and Adam Clayton Powell Jr. Boulevard.

Sommerville recalled growing up in Birmingham and getting jailed at age 16 for taking part in the May 1963 civil-rights demonstration that shocked the nation when Sheriff Bull Connor used fire hoses and police dogs to disperse a peaceful protest by local high-school students.

“Hearing the bell toll today, it was sad,” Sommerville said.

“A lot of these young kids don’t know nothing about what we went through then.”

Also yesterday, about 100 people marched through Harlem to The Bronx to protest the slaying of Florida teen Trayvon Martin, whose killer, George Zimmerman, was acquitted last month.